DES MOINES, Iowa - Republicans in search of their party's presidential nomination are returning to campaign mode after a brief Christmas respite, with Rick Santorum planning a hunting trip with conservatives in Iowa and Mitt Romney phoning supporters.

With just a week until Iowa holds its leadoff caucuses and many caucus-goers undecided, the final push ahead of the Jan. 3 contests was heading into a critical time. Campaigns planned new television ads and phone calls to persuade holdouts still weighing their options.

Romney, who kept this state at arm's length for most of the year, seemed to increase his efforts in Iowa as polls found him in a stronger position. He planned to talk with supporters in a series of telephone calls here and to New Hampshire and Florida on Monday between working on a speech that aides described as his final pitch to Iowans. Romney planned to deliver that speech Tuesday evening and then set out on a bus tour of Iowa.

However, he was to share the highways with Rep. Michele Bachmann, Texas Gov. Rick Perry and former House Speaker Newt Gingrich. All scheduled bus tours to start then, too.

Each is running out of time and looking to derail Rep. Ron Paul, the Texas congressman who seems to have the most sophisticated network of volunteers ready to organize for the caucuses. Paul was to return to Iowa this week to meet with supporters he has kept in touch with since his unsuccessful run in 2008.

Others, too, were ready to turn on their own political machines and put fresh ads on the air.

Gingrich, who last week criticized the negative tone of the campaign, was preparing to directly challenge Romney on the economy, an issue Romney has made central to his campaign. Gingrich's standing in public and private polls has slipped as he faced unrelenting criticism from the candidates and their allies.

Gingrich was expected to use clips from Romney's previous campaigns distancing himself from President Ronald Reagan and pitch his own economic plan as "Reaganomics 2.0." Gingrich also was expected to compare Romney's tax plan with his own.

Romney released a new TV ad in Iowa on Monday that touts him as a conservative businessman and in which he says "it is a moral imperative for America to stop spending more money than we take in."

"It's killing jobs and it's keeping our kids from having the bright prospects they deserve," he says in the spot. "The experience of balancing budgets is desperately needed in Washington and I will take it there."

Santorum, meanwhile, planned to announce support from another wave of Iowa conservatives. He scheduled a pheasant hunting trip in Adel for Monday afternoon. While he trails in polls and has not spent significant money on ads, Santorum is hoping his nonstop courtship of Iowans yields a late surge. He visited all 99 of Iowa's counties during the summer - an accomplishment Bachmann has feverishly tried to replicate.

Bachmann, a congresswoman from Minnesota, last week darted through small towns, reminding voters that Santorum lost his 2006 re-election bid in a blowout and that Paul's foreign policy views were outside the party's orthodoxy.

Looking to recapture voters' interest, her plan was to return to hand-to-hand campaigning Tuesday and paint herself as the only acceptable conservative in the race.

"You can always count on me to stay true to my word and put America and Americans first," she said in a fundraising email sent Monday. "I am the consistent conservative who will put our nation back on a path towards prosperity and restore our values to government."

Perry, too, was looking to keep up the message that his rivals are insiders unable to change Washington. He planned to resume his tour bus on Tuesday.

Former Utah Gov. Jon Huntsman kept his focus on New Hampshire. Huntsman said early in the campaign that he would not compete in Iowa and instead make his start in New Hampshire, which comes second on the nominating calendar with a Jan. 10 primary.